Electrical amplifier circuit



Mar-ch 1931. P, o. FARNHAM ET AL ELECTRICAL AMPLIFIER CIRCUIT 2 Sheets-Sheet 1 Filed June 8. 1929 gwue'ntofo: (2%

. March 31, 1931.

P. O. FARNHAM ET AL ELECTRICAL AMPLIFIER CIRCUIT 2 Sheets-Sheet 2 Filed June 8. 1929 500 Mer r Elrcmm a.

Patented Mar. 31, 1931 UNITED STATES PATENT OFFICE PAUL O. FARNHAM AND RAYMOND ASSERSON, OF BOONTON, NEW JERSEY, ASSIGNORS TO RADIO FREQUENCY LABORATORIES, INCORPORATED, OF BOONTON, NEW JERSEY,

A CORPORATION OF NEW JERSEY ELECTRICAL AMPLIFIER CIRCUIT Application filed June 8, 1929. Serial a... 369,286.

This invention relates to electrical amplifier circuits, and more particularly to circuits includin elements for automatically varying the e ective amplification at differ- 51 ent frequencies.

In audion amplifiers of the type in which tuning is effected by varying the capacity across a .fixed' inductance thegain is not uniform over a band of frequencies but increases and particularly so in broadcast receivers and other cascaded ampli in systems. If a gain-wave length curve is p otted for the usual transformer coupled radio frequency amplifier stage, or for cascaded stages, the curve will have a relatively steep slope, the amplification at the higher frequencies within the broadcast range being approximately twice the amplification at the lower frequencies in the case of a triode and being several times as great in the case of a'tetrode of the shield grid type.

An object of the present invention is to provide an electrical amplifier circuit in which the amplification rate may be approxi-' mately constant throughout a given range of frequencies. A further object is to provide an amplifier circuit in which the effective amplification is decreased, over a predetermined portionof the range of frequencies to which the amplifier may be tuned, by the radio frequency voltage drop established across a'portion of the Input circuit by amplified signal energy flowing in the output circuit.

More particularly, an'object of the invention is to provide an electrical amplifier circuit in which a radio frequency impedance is connected between the cathode and the junction of the input and output circuits, the impedance being so proportioned that it is resonant at a frequency at or near the lower limit of the band of frequencies whichis to be amplified.

.These and other objects of the invention will be apparent from the following description when taken with the accompanying drawings in which:

Fig. 1 is a circuit diagram of an amplifier stage embodying the invention;

Figs. 2 and 3 are circuit diagrams illustraing other embodiments of the invention; an

Fig. 4 illustrates typical gain-wave length characteristic curves which will be hereinafter explained.

In Fig. 1, the invention is illustrated as embodied in a balanced radio frequency amplifier stage of the type described in the patent to Hull, No. 1,672,811, June 5, 1928. The tuned input circuit L G is connected between the grid G and the point Y in the cathode circuit to which the lower terminal of the plate impedance L is also connected. The tuned output circuit which feeds the succeeding stage comprises the'serially connected coils L L and condenser G The coils L and L constitute a coupling transformer, while the coil L has zero or negligible coupling with the-other coils, the balance condenser C being connected between the grid G and the junction of the coils L L When the circuit includes only the elements which have been described above, the amplification rate or stage gain is materially higher at the shorter wave lengths than at the longer wave len hs. The novel construction which modifies the stage gain comprises the impedance Z, which is serially connected between the cathode and the junction Y of the-input and 1 output circuits. The radio frequency by-pass condenser C is connected across the plate current suppl B but it is to be noted that the condenser G oes not by-pass impedance Z Theimpedance Z is so chosen that radio. frequency potential drop across the impedance is greater at the higher than at the lower frequencies. It will be apparent that, due to the potential drop across the impedance Z the effective signal voltage between grid and cathode will not be the voltage across the resonant input circuit L C The iInpedaneZ may of coursegbe so chosen thatthe stage gain '[may be/"d e- ,creased over .anydesiredportion of the mon portion of the input and output circuits lies between the ground or sh1eld and the cathode.

This common portion of the cathode circuit includes the resistor R shunted by the composite impedance formed by inductance L in series with condenser C The resistance It, is the usual bias resistance and its value 15 so chosen that the direct current potential drop across the resistance provides the negative bias for the grid. As usually employed, the bias resistance is shunted by a. by-pass condenser of the order of one-half m. f. which affords a path of negligible impedance for all frequencies within the range to be amplified.

In accordance with the present invention, the usual by-pass condenser is replaced by the series resonant circuit L G which corresponds to the generic impedance Z of Fig. 1, and the relative values of the impedances L and C are so chosen that the series circuit is resonant at or near the maximum wave length to whihh the stage may be tuned.

The automatic gain control is e ually appllcable to amplifier stages embo ying tetrodes of the shield grid type. The amplier stage which is diagrammatically represented in Fig. 3 includes tuned input circuit L C a tuned output circuit L G and circuit connections for placing the desired positive potential upon the screen grid G Except for the provision of the impedance in that portion of the cathode circuit which is common to the input and output circuits, the design and construction of-the amplifier stage may be of any approved or desired form. The impedance is provided by the inductance L 1n series with the condenser G which shunts the bias resistance R As applied to tetrodes, the present invent1on decreases the stage gain at short wave lengths wlthout appreciable loss in selectlvity, and therefore the construction is superior to those previously employed for securing an approximately uniform gain over the frequency range of an amplifier.

The curves of Fig. 4 are representations of the gain-wave length characteristics; of a conventional amplifier stage and amplifier stages embodying the invention. Curve A represents the erformance of a balanced amplifier stage 0 the type shown in Fig. 2, but in WhlCll the resistance R was shunted by a 1 m. f. condenser. It will be noted that the gain at 200 meters was approximately double the gain at 600 meters. Curves B and G represent the measured gain of the same stage when the usual by-pass condenser was replaced by the series resonant circuit L G The coil L had an inductance of approximately 55 microhenries and the capacity of G was chosen for resonance of the impedance Z, at 440 and 500 meters, respectively, that is, curve B was obtained when G was .001 m. m. f. and curve C was obtained when G was .0014 m. m. f.

Attention is calledto the fact that the gain is substantially unchanged for the frequencies at which the impedance Z,; is resonant and that the gain for lower frequencles is higher than the gain obtained when the bias resistance was-shunted by the usual bypass condenser. At frequencies above the resonant frequency of the impedance Z,, the stage gain is substantially less than that of the usual circuit arrangement.

It will be apparent that the impedance Z may take the form of more or less complex impedances of a compositenature when it is desired to change the gain over a limited portion or portions of the wave-length range. In the design of broadcast receivers, however, the use of complex impedances is unnecessary, since the general fiattening of the curve which may be obtained with a simple series circuit will usually meet the requirements when several stages are to be cascaded.

While the invention may be employed in amplifier stages of the conventional type as well as those which are balanced to prevent self-oscillation, it is particularly useful with such balanced stages, since the general balance conditions of the stage are not diturbed by the inclusion of the gain control impedance Z Althoughthe invention has been specnlcally described in connection with stages designed for operation within the broadcast range, it will be apparent that the invention is applicable generally to all amplifier stages or cascaded stages. In an audio frequency amplifier stage, for example, the impedance Z; may be so chosen as to efiect a cutoff at or near adisturbing frequenoy.;-- The resonant impedance Z, is efiectively in shunt across the bias resistor R even when the inductance L is in series with the resistor since the direct current resistance of L is negligible. 7

We claim:

1. In an amplifier adapted to be tuned over a band of frequencies, the combination with 'an audion having a control electrode, a cathode and an anode, of an input circuit connected between control electrode and cathode, an output circuit connected between cathode and anode, and a circuit element constituting a series impedance common to both said input and said output circuit, said circuit element being resonant at a frequency at the lower limit of the said band of frequencies.

2. In an amplifier adaptedfor operation over a band of frequencies, the combination with an audion having a control electrode,

.a cathode and an anode, an input circuit and an output circuit, of means for automatically decreasing the effective amplification over a portion of said band, said means comprising an impedance element connected between said cathode and the junction of said input and output circuits, the constants of said element being such that its impedance is greater at frequencies falling within said portion of the hand than for frequencies lying outside of the same.

3. An audion amplifier of the type including an audion havin an input andan output circuit, and a variable condenser for tuning one of said circuits over a band of frequencies, characterized by'means for decreasing the variation of amplification with changing wave length, said means including an impedance connected between said cathode and the junction of said input and output circuits, said impedance being reactive and having an impedance which, within the frequency range of said amplifier, increases at a substantlal rate with increasing frequencies.

4. In an audion amplifier, adapted to be tuned over a band of frequencies, the com- 7 bination with an audion having a control electrode, a cathode and anode, an input circuit. and output circuit including a source of direct current, and a grid bias resistor in a common portion of said input and output circuits, of an inductance and condenser forming a series resonant circuit effectively in shunt across said resistor, the values of the impedances in said series circuit being so related that the circuit is resonant at the lower end of said band of frequencies.

5. In an audion amplifier, adapted to be tuned over a band of frequencies, the combination with an audion having a control electrode, a cathode and anode, an input circuit, an output circuit including a source of direct current, a grid bias resistor in a common portion of said input and output circuits, and a condenser by-passing said resistor, of an inductance cooperating with said condenser to form a resonant circuit in said common portion of said input and output circuits, the impedances of said'inductance and condenser being so related that thecircuit formed thereby is resonant at the lower end of said band of frequencies.

In testimony whereof, we afiix our signatures.

PAUL O. FARNHAM. RAYMOND ASSERSON. 

